Biocompatibility is a poorly understood area. Implanted materials typically induce an immune response which can lead to tissue capsule formation which can lead to failure of medical devices. In this proposal, we plan to develop a method for monitoring biocompatibility in vivo. The imaging method, which monitors neutrophils and macrophages, that we propose to develop will be validated with histology. Similar trends between histology and in vivo imaging such as the days in which the phagocyte populations reach a peak will confirm the validity of this technique. Biocompatibility of many materials will be examined through the use of this in vivo imaging technique. The first experiment will examine how macrophages respond to different sizes, shapes, and surface functional groups. Next, biodegradable materials of different compositions and sizes will be examined which have applications in drug delivery systems, orthopaedic fixing devices and scaffolds for soft and hard tissue repair. The final goal of this project is to study a novel library of polymers through in vivo imaging and histology. The in vivo data will be compared to surface analysis of the mechanical properties and the hydrophobicity of the materials. This study will lead to an understanding of biocompatibility of materials and aid in rational design of future generations of biomaterials.